Beresford entered the British Army in 1785 as an ensign in the 6th Regiment of Foot and the next year he was blinded in one eye due to an incident with a musket. He remained in the service being promoted to captain by 1791 with the 69th Regiment of Foot. He distinguished himself at Toulon (1793), in Egypt (1799–1803) and in South Africa (1805). From there he fared across the South Atlantic to South America to invade the River Plate region (now Argentina), with a small British force of 1,500 men, departing on 14 April 1806. Following his move to Cape Town in Cape Colony, Beresford, spurred on by Home Popham, R.N. (later Rear AdmiralSir Home Popham), decided to attack Buenos Aires in Spanish South America. No attempt was made to gain authorization from the Crown for this undertaking. In the invasion of the River Plate, Buenos Aires was occupied for 46 days. However, the British force could not maintain itself against the army gathered by Santiago de Liniers. After a relentless two-day fight with the Buenos Aires and Montevideo militias between 10 and 12 August 1806, the British were defeated and forced to capitulate. Beresford had to surrender, remaining prisoner for six months; in the end, he managed to escape and arrived in England in 1807.[2]

In that same year Beresford was sent to Madeira, which he occupied in name of the King of Portugal, remaining there for six months as Governor and Commander in Chief. The exiled Portuguese Government in Rio de Janeiro, Brazil, whereto the Portuguese Royal Family had transferred the Court, realised the necessity of appointing a commander-in-chief capable of training, equipping and disciplining the demoralised Portuguese Army. The Portuguese government asked Britain to appoint Arthur Wellesley to this role, Wellesley indicated he could not do the role justice due to his prior engagements and recommended Beresford. He was appointed Marshal and Commander in Chief of the Army by Decree of 7 March 1809 and took the command on 15th of the same month. At that time, Marshal Soult had already crossed into Portugal where he occupied Porto. Beresford quickly overhauled the Portuguese forces, bringing them in line with British discipline and organization, and from the General Headquarters (then at the Largo do Calhariz), he dispatched many "daily orders" altering points of the Infantry ordnance, creating a general command of Artillery, establishing the separation of the battalions, firing incompetent or corrupt officers and promoting or appointing appropriate replacements.

On 22 April 1809 Sir Arthur Wellesley, later Duke of Wellington, disembarked in Lisbon, and took over the command of all the Anglo-Portuguese troops and he was nominated Marshal General of the Portuguese Army. The allied armies marched to the North. Wellington moved from Coimbra directly to Porto, which he entered on 12 May, and Beresford marched through the Province of Beira, arriving that same day at the banks of the Douro river, in the area of Lamego. Wellington's troops made a forced crossing of the Douro and defeated the French, Soult was obliged to withdraw from Porto. Soult was outnumbered and was expelled from Portugal; the positioning of Beresford's forces compelled the French to leave Portugal by the poor roads through Montalegre, they managed to cross the border only after sacrificing their artillery and baggage, and facing numerous difficulties.

The Second French Invasion of Portugal was defeated and the allied armies moved back to the South, the British concentrating at Abrantes and the Portuguese at Castelo Branco. with the intent of cooperating with the Spanish against Marshal Victor, the Anglo-Portuguese forces under Wellesley moved into Spain in the Talavera campaign while Beresford remained on the Águeda River covering the Spanish-Portuguese border. After Wellesley's return, now as Viscount Wellington, following the Battle of Talavera Beresford re-entered Portugal, where he distributed the army at various locations established his General Headquarters in Lisbon. From Lisbon he dispatched numerous orders and instructions for the reform of the Portuguese military.

In the same year (1809), and the following year he made tours of inspection of the corps that were found quartered in the various provinces and he corrected any defects he noticed and established rules for the functioning of the different branches of the military service. In this way he improved the functioning of the Portuguese Army so that they might face the forces of Napoleon that were invading the country for the third time. The good results of his efforts were proven at the campaign against Masséna in particular at the Battle of Buçaco, 27 September 1810 where the Portuguese troops played a prominent part, and also in the defence of the Lines of Torres Vedras).

The most notable action in which Beresford held independent command occurred in 1811 when a combined Anglo-Portuguese and Spanish army under his command, intercepted a French army commanded by Marshal Soult who had been ordered by Marshal Auguste Marmont to move to protect the important Spanish fortress-city of Badajoz. As the French forces retreated from the Lines of Torres Vedras, Beresford marched towards Badajoz, which he laid siege to. Having, however, received notice that Soult was approaching, he lifted the siege and posted his army at Albuera in a defensive position. There he defeated the French forces on 16 May 1811. After the very bloody Battle of Albuera the French were forced to retreat, though the siege of Badajoz had to be subsequently abandoned. Meanwhile, on 13 May 1811, he was created Count of Trancoso in Portugal by decree of Prince Regent John.

At the beginning of July 1811, he was again in Lisbon, but he was subjected to fits of "nervous breakdowns", as described by brigadier D'Urban, quarter master general of the Portuguese Army. He recuperated in February 1812 and he then joined Wellington in his investment of Ciudad Rodrigo. He accompanied, after this fortified town had fallen, the army to Alentejo, and participated in the Anglo-Portuguese Siege of Badajoz. After Badajoz had been stormed the two Generals, Wellington and Beresford, again took up position on the Águeda, and from there launched the Salamanca campaign. On 22 July 1812, the important Battle of Salamanca was fought, giving the Anglo-Portuguese forces a decisive victory over the French under Marshal Marmont. In the battle Beresford was badly wounded, under his left breast, when he was ordering the advance of one of the Portuguese brigades.

He retired to Lisbon and had bouts of fever and was half incapacitated for several months until May of the next year (1813). Meanwhile he was also created Marquis of Campo Maior in Portugal by Prince Regent John on 17 December 1812. In March he was confirmed as second in command of the Allied Army and rejoined the campaigning army, and assisted in the liberation of Spain by the British and Portuguese armies.

After peace was declared he went to England on leave and came back again to Lisbon to reassume the command of the Portuguese Army. He did not limit himself, however, to that role, and intended to intervene in the general politics of the country, from this he came into conflict with the Regency. He then determined to go to the Court in Rio de Janeiro. He departed to there in August 1815 and returned in September 1816, invested with wider powers than the ones which he had previously enjoyed. Beresford took a high hand in his dealings with Gomes Freire de Andrade (1817) and, put into a difficult situation, he returned to Brazil, obtaining from John VI the confirmation of the powers he had already attained, which he desired to see amplified.

When he returned to Portugal, the Liberal Revolution of 1820 intervened; the British officers, for the most part, had been discharged, and the government would not even consent that Beresford could disembark. He was made Governor of Jersey in 1821 and held the position till 1854. Briefly returning to Portugal in 1827 at request of the Regent, Infanta Isabel Maria of Braganza, he gave up his ambitions due to the resistance he found among the new Portuguese elite and returned to Britain.

William Beresford, 1st Viscount Beresford, was the last titular Governor of Jersey

Napier, in his History of the Peninsular War, severely criticized the tactics of Beresford at the Battle of Albuera, which gave origin to a heated correspondence between the Marshal and the historian. The published letters of Beresford which are mentioned below refer to this controversy. Wellington himself had no illusions over Beresford's ability as a General, but he appreciated his abilities as a military organizer that he recommended, in case of his death, that Beresford succeeded him in command. He published: Strictures on Certain Passages of L. Col. Napier's History of Peninsular War; Further Strictures; Refutation of Col. Napier's Justification, London, 1831–1834, 3 Vol.; Letter to Charles Edward Long, Esq. on the Extracts Recently Published from the Ms. Journal and Private Correspondence of the Late Lieut-Gen. R. B. Long, London, John Murray, 1833; A Second Letter to Charles Edward Long, Esq. on the Ms. Journal and Private Correspondence of the Late Lieut. General R. B. Long (1834). Also of interest is the Colecção das Ordens do Dia (Collection of Orders of the Day) produced by Beresford's general headquarters nos Anos de 1809 a 1823 (for the years 1809 to 1823), Lisbon, 13 Vol. (at the Library of the English Institute of the University of Coimbra).

Lord Beresford married his first cousin the Honourable Louisa, widow of Thomas Hope and daughter of William Beresford, 1st Baron Decies and Elizabeth Fitzgibbon, in 1832. The marriage was childless. She died in July 1851. Lord Beresford died in January 1854, aged 85. The barony and viscountcy became extinct on his death. His estates were passed on to his stepson, Alexander Beresford Hope.[1] Beresford's Portuguese titles were not renewed, although his nephews continued using them.[citation needed]